I highly recommend this series of articles (now 4) on nitrogen by Ian Angus:
https://climateandcapitalism.com/2019/10/25/disrupting-the-nitrogen-cycle-articles-on-a-major-metabolic-rift/
(These are part of a larger series on "metabolic rifts": how humans have disrupted natural cycles.)
Joshua, your use of the term "fixed" is confusing to me. I have seen it used only in reference to the soil bacteria that take in N2, atmospheric nitrogen gas, and convert it ("nitrify") to plant available form in the soil. Basically, as I understand it, these bacteria take in the N2 and use it for their own body processes, and for those bacteria with a symbiotic plant relationship, offer it as nitrate of ammonium to the partner plant. Excess N is excreted so is then available to other plants.
The danger of excess, human applied N, is that it is leached into groundwater where it becomes a pollutant, or is released into the atmosphere as Nitrous Oxide, a potent greenhouse gas. There may be other ways excess N pollutes that I'm not yet aware of.